I don't think Allies are underpowered. I think Soviets are. Soviets depend heavily on Iron Curtain and Tech Centre. Without the IC they have no chances against Allies and a lot of successful attacks/breakthroughs depend on IC'ed units. In terms of different strategies Allies can do everything Soviets can, and they additionally can control the sea and drop units.
The most powerful unit in the game, Tanya, belongs to Allies. Base pushes of Allies are much stronger due to the fact that you can build turrets faster than tesla coils. Spies can absolutely win you games, and keeping an eye on them is a constant pressure, so you will miss at least one or two (even best players don't always notice them). Allies have medics and mechanics, so if you're lucky enough you can micro properly and retain a lot of your army. Soviets are doomed to lose more because arty + Tanya leaves 0 chances to infantry (even to shock troopers) and tanks on their own can't do anything (except maybe tesla tanks).
I don't get why everyone is disappointed with Chronosphere. It's pretty damn powerful already. You can: destroy enemy harvesters, teleport arties, teleport tanks, steal enemy units, crush infantry, teleport MCV's. So many uses!
Spies should be given to solely Britain
I think it was my idea and I still like it. Transfer spies to Britain and make them worth $500. It's an extremely powerful unit and the price of $250 is just a joke. Hijackers cost $500 (IIRC) and they're buggy. Spies, on the other hand, always work reliably.
Mammoths are underpowered
Indeed they are. They cannot use their ability to regenerate health at all because they can *never* escape due to their slowness. I would increase their speed leaving the turn rate the same and boost splash damage of tusk rockets. I'd love them to be similar to TD mammoths, but less powerful. So, it should be something in between the current mammoth and the TD mammoth.
Allied late game is underwhelming, something to make allied tech have a bigger difference should be introduced
It's already much more versatile than what Soviets have (given so many different special units) and GPS satellite provides ultimate vision, so sneak attacks are no longer possible. It's hard to keep it alive for 9 minutes? Hah, build two! Chronosphere is amazing, too. But it should not whiff that often and teleport nothing.
longbows are underpowered (too slow, dont do enough damage vs armor)
I would *only* increase their damage, just a bit, so they could recharge less often.
Migs should have anti-air capabilities
I think this will break the game. Migs should definitely be faster to build (the price is probably reasonable) and have slightly more health (flying tanks). As of this release you need at least 4 migs to destroy 1 AA-gun in 1 go , and you lose 1 mig in 100% of cases. This is ridiculous given the price of the unit vs AA-gun. Ideally, in this scenario, you should lose no migs and increased health can provide this.
Missile subs should have an anti-air capability
I think everyone likes this so far.
gunboats should have an anti-navy role
I like this idea, and it also requires a change to destroyers, so they could not attack subs. Destroyers - land/air damage. Gunboats - sea.
More untis/buildings should detect spies
Make silo reset the disguise
Just kidding, I don't know what to say here.